Integrins are transmembrane glycoproteins that mediate cell-cell and cell-matrix interactions. They contain two subunits, .alpha. and .beta., which are joined in a non-covalent complex. There are numerous .alpha. and .beta. subunits known. Alpha subunits show some homology with other alpha subunits and beta subunits tend to show homology with other beta subunits, however, the alpha subunits tend to be quite distinct from beta subunits.
Osteoclasts are the primary cells responsible for bone resorption. Osteoclasts migrate to the area of the bone to be absorbed, and then attach to the bone. Adhesion molecules, including integrins, are believed to be involved in the processes of migration and attachment.
Recent studies have shown that both mature osteoclasts and tissue culture generated osteoclast-like cells highly express the vitronectin integrin receptor .alpha..sub.V .beta..sub.3. The .alpha..sub.V .beta..sub.3 integrin receptor recognizes the tripeptide Arg-Gly-Asp (RGD), found in many bone matrix proteins, and thus is thought to be involved in the attachment processes. However, there is no direct evidence that .alpha..sub.V .beta..sub.3 mediates osteoclast attachment to bone in vivo.
Partial sequence of the mouse .beta.3 cDNA was previously reported by Cieutat, et al., 1993 Biochem. Biophys. Res. Comm. 193:771-778. Cieutat et al., cloned .beta.3 from mouse kidney RNA using RT/PCR and human primers. This published sequence did not have the N-terminus and the last 4 amino acids at the C-terminus.
There are presently two types of screens for the .alpha..sub.V .beta..sub.3 ligands as an inhibitor for bone resorption: a binding assay based on human recombinant .alpha..sub.V .beta..sub.3 integrin and a functional assay based on rodent osteoclasts. To exclude the possibility of species-based potency differences in ligand interaction with the .alpha..sub.V .beta..sub.3 integrin, it would be desirable to develop an assay which uses the .beta.3 integrin subunit from a mouse osteoclast.